During the past few years there has been a substantial growth in the production of high-strength paper and cloth products having a nonwoven, randomly-oriented structure, bonded with a polymeric resin binder. Such products are finding wide use as high-strength, high-absorbency materials for disposable items such as consumer and industrial wipes or towels, diapers, surgical packs and gowns, industrial work clothing and feminine hygiene products. They are also used for durable products such as carpet and rug backings, apparel interlinings, automotive components and home furnishings, and for civil engineering materials such as road underlays. There are several ways to apply a binder to these materials, including spraying, print binding, and foam application. Further, depending on the end use, various ingredients such as catalysts, cross-linkers, surfactants, thickeners, dyes, and flame retardant salts may also be incorporated into the binder.
In the high-speed, high-volume manufacture of cellulosic products such as wet wipes, an important binder property is a fast cure rate; i.e., the finished product must reach substantially full tensile strength in a very short time after binder application so that production rates are not unduly slowed down. In these products, such a property is usually obtained by using a binder which is either self cross-linkable or by incorporating an external cross-linker into the binder formulation. The cross-linker or self cross-linkable binder apparently not only interacts with the binder monomers but with the hydroxyl groups on the cellulose fibers as well to quickly form very strong bonds.
As the need for stronger nonwovens developed, a variety of cross-linking agents for the base binders was utilized. N-methylolacrylamide and other similar cross-linkers were incorporated into the binders. While the strength of the nonwovens increased desirably, it was discovered that many of these cross-linking agents, especially N-methylolacrylamide and similar materials, emitted formaldehyde during use. The toxicity of formaldehyde caused users to search for non-formaldehyde emitting alternatives. An example of a non-formaldehyde emitting cross-linker is methyl acryloamidoglycolate methyl ether (MAGME). However, while MAGME improved the strength of many copolymeric binders and did not emit formaldehyde, the need for further improving the strength, especially the wet strength of many copolymeric binders, led to the use of various other techniques for strength improvement.
One method of providing a fast curing, "zero" formaldehyde binder for nonwoven cellulosic materials utilized a binder comprising a solution copolymer formed by reacting a mixture of two or more water soluble olefinically unsaturated organic comonomers. The solution copolymer was admixed with a non-formaldehyde emitting latex to produce a final composition which, when cured on nonwoven cellulosic material, achieved about 80 percent of fully cured wet tensile strength in 8 seconds or less and which had essentially no emission of formaldehyde from the finished nonwoven.
While this approach resulted in providing zero formaldehyde emitting binders which had improved wet strengths and which were capable of fast curing, it has been found that solution polymers may raise the viscosity and cause thickening of the binders in which they are incorporated. While the viscosity may be varied within certain ranges, in certain applications it is desirable to maintain the viscosity of the binder at a relatively low level in order to assure adequate penetration of the binder into the nonwoven substrate. Accordingly, a method of providing high strength nonwovens which does not fully depend upon the incorporation of large quantities of solution copolymers was needed.